I think the 300k is a rough estimate of current holdings based on last week's interest payment.
It appears that at least 22k BTC was paid out in interest last week. Assuming nobody automatically re-invested, and that everyone gets 7% per week, that means total holding are 22k/0.07 = 314k BTC. Of course some people do re-invest, and some get lower interest rates, so the real figure will be higher than this.
My guess is that many of his clients, probably a majority, work with compound interest. My reasoning is that people who pay in a relatively small amount base their investment on the hope to turn their insignificant initial investment into a significant amount.
If you only pay in a three-digit amount, what you earn by having your 7% paid out weekly is and remains peanuts. But if you could compound your interest over a year or two or three, you'd be rich.
I therefore guess that those who compound their interest tend to be three-digit investors, while those who collect their interest weekly tend to be those with larger investments. This is admittedly only a wild guess. It would mean that he has considerably more than 300k BTC, but not two or three times as much. My rough guess (based on your guess) would be 500k, quite a tidy amount of untidy money, enough to live a moderate life of leisure. It remains mere guesswork, but I wouldn't be surprised if we hit near home.
This makes me more confident that the scheme will end soon, probably this month, next month at the latest. If he kept having to pay out 22k BTC every week, without a steady stream of new investors his stockpile would be empty after a relatively low number of weeks, much less than a year. But he does not want to get anywhere near empty, so he has to end it very soon.
If he wanted to launder most of his gains, his laundromat would have to be running full bore now. Is that what we actually observe?